ISIS is the headline threat this week, but evidence is growing daily that an even bigger danger looms: a nuclear Iran.

And President Obama isn’t just shutting his eyes to it, as he did with ISIS, but enabling it.

The latest sign: Washington’s rush to close the book on Iran’s past nuclear work, even though Tehran flatly refused to answer key questions posed by UN monitors. Washington will reportedly vote Tuesday, with five other nations, to end the probe of that work — incomplete as it is.

That’s a huge flip. Secretary of State John Kerry once insisted the mullahs come clean on their past activities: “They will have to do it,” he said. “If there’s going to be a [nuclear] deal, it will be done.”

But when Tehran refused to go along, Kerry caved. It was more important, he said, to get a deal (any deal) than to have Iran ’fess up.

Sirens should have wailed right there. Because without full knowledge of what Iran’s been up to, it will be impossible to know if it’s truly abandoned its quest for nukes, as the deal requires.

As it is, monitors uncovered enough evidence to show Iran had been working to build nukes right up until 2009 — far longer than anyone thought.

And Tehran’s failure to come clean is troubling for another reason: After all, if it won’t cooperate with monitors before sanctions are lifted, why on earth would it do so afterward, when it’s already pocketed the hundred billion-plus dollars promised in the agreement?

No, it’ll just cheat. If it gets caught, it has little to fear of any “snap back” of sanctions.

Need more proof? On Monday, two US officials said Iran tested a new medium-range missile last month, in violation of a longstanding Security Council resolution. The missile has a range of 1,180 miles (far enough to reach Israel) and can carry nukes. A similar test in October drew no consequences.

ISIS may well prove to be the least of the horrors Obama’s leaving to his successor.